At his first campaign rally of the 2018 election cycle, former President Barack Obama attempted to impress upon California Democratic loyalists how urgent it is that they vote in November, but Republicans are banking on their base getting the same message from the rare attack on a current president by his predecessor.

"This is a consequential moment in our history. The fact is if we don't step up things can get worse,” Obama said in Anaheim as he stumped for seven Democratic candidates running against Republican incumbents in districts that Hillary Clinton won in 2016. “But the good news is in two months, we have a chance to restore some sanity in our politics. We have a chance to flip the House of Representatives."

While Obama’s comments about threats to democracy and political division Saturday seemed to be aimed at President Donald Trump, he never mentioned the current White House occupant by name. A day earlier, at an event in Illinois, he targeted Trump more explicitly than he had since the end of the 2016 campaign.

"It did not start with Donald Trump, he is a symptom, not the cause. He is just capitalizing on resentment that politicians have been fanning for years, a fear, an anger that is rooted in our past but is also borne in our enormous upheavals that have taken place in your brief lifetimes," Obama said at the University of Illinois at Urbana-Champaign.

It is not yet clear how often the former president will be seen on the campaign trail this fall or how much of his messaging will be focused on Trump. Obama will appear in Ohio Thursday at an event for Democratic gubernatorial candidate Richard Cordray, and he is scheduled to speak in Pennsylvania later this month.

“President Obama's campaign appearance in California is all upside for Democratic congressional challengers,” said Darry Sragow, a longtime California Democratic strategist. “A primary purpose of his visit is to increase Democratic turnout in districts that historically have voted Republican.”

Considering how vital former Obama voters were to Trump’s victory in 2016, he is well-positioned to encourage those voters to switch back, according to David Ormsby, an Illinois public relations consultant and former press secretary for the Democratic Party of Illinois.

“The former president can speak uniquely to the demographic who voted for both him and Donald Trump, aiming to persuade those swing voters with buyer's remorse to return to the Democratic column in November," he said.

For a Democratic base that has been marching through the streets in outrage over Trump’s presidency, Obama is a welcome, if belated, conscript to the resistance.

“There in some ways had been a certain amount of frustration given what has been happening over the last 20 months he hasn’t been out there,” said Neil Sroka, communications director for Democracy for America, a progressive political action committee that endorsed Bernie Sanders in the 2016 primaries. “The forcefulness and the directness of his comments I think is broadly in line with where many parts of the grassroots base are on this election.”

“That was something a lot of people noticed and appreciated,” Sroka said. “It’s a good reflection of the fact that former President Obama seems to be responding to the changes happening in the political landscape.”

Republicans were also happy to see Obama embrace some of the more liberal policies on the Democratic agenda.

“As long as Barack Obama is talking Medicare-for-all, he’s pushing independent voters into the Republican camp,” said Mark Weaver, an Ohio-based GOP strategist.

According to Jason Mollica, a former journalist and communications professional who now teaches at American University, Obama and other prominent Democrats must be cognizant of Republicans who are reluctant to support Trump but struggling to find a palatable alternative.

“They want to make sure they’re putting forth messaging that’s going to work for Americans who have a difficult choice,” he said.

It remains to be seen whether Obama can thread that needle, but his first foray into the midterm battle drew aggressive counterattacks and cries of hypocrisy from Republicans.

“If he’s right and Donald Trump is the symptom of a cause, well, Donald Trump got elected in 2016 after eight years of Barack Obama as president. He can’t detach himself,” former New Jersey Gov. Chris Christie said on ABC’s “This Week.”

Vice President Mike Pence called Obama’s attacks “very disappointing” in an interview with CBS News, but Republicans are hoping his highly-publicized speeches will continue to remind GOP voters why they supported Trump in the first place as the election approaches.

According to Sroka, the risk of inadvertently galvanizing Republicans is overstated and is far outweighed by the boost Obama can provide for Democratic candidates.

“Honestly, it’s hard to imagine an individual or activist that wasn’t already supporting Donald Trump’s bigoted agenda being drawn into the process because Barack Obama spoke up for decency and democracy,” he said.

Much like Trump is campaigning in favorable districts and staying out of Democratic strongholds, Mollica expects Democrats will deploy Obama strategically to minimize the blowback.

“President Obama is very smart and he’s not going to just go out there and fly off the handle,” he said.

A recurring theme in Obama’s comments was the importance of getting out and voting this fall. Weaver questioned whether delivering that message is worth the potential Republican backlash.

“If the Democrat base were flagging or there was a lack of enthusiasm among Democrats for voting, then Obama running around taking potshots at the president might help, but Democrat voters are already planning on voting,” he said.

Sroka fears that perception could make Democrats complacent.

“Certainly, Democrats are enthusiastic,” he said, “but I think there is broad agreement we cannot take one single shred of that enthusiasm for granted.”

Republicans have portrayed Trump’s election as a repudiation of Obama’s presidency, but Obama had an approval rating nearing 60 percent when he turned over the Oval Office. His final rating in Gallup’s daily tracking poll was higher than it had been since mid-2009. As with most things in modern politics, though, opinions on Obama are highly partisan, and there is no doubt his speech stirred anger within the GOP base.

At a campaign event Friday afternoon, Trump joked that he had fallen asleep during Obama’s speech. His predecessor’s criticism clearly stung, though, leading Trump to tweet out rebuttals over the weekend. Kevin Hassett, chairman of the White House Council of Economic Advisers, was brought in to brief the press Monday in what appeared to be an effort to refute Obama’s assertion that he bears some responsibility for the health of the economy.

“President Trump has achieved clear success in jump starting the economy, defying repeated attempts to credit the Obama Administration,” a fact sheet distributed by the White House Monday claimed.

As he promised before leaving office, Obama has never fully receded from the national stage. He has issued statements rebuking Trump, albeit somewhat obliquely, on several occasions defending his legacy on issues like the environment, health care, and immigration.

“It’s kind of bad form for a retired president to wade back into politics,” Weaver said. “In some churches, there’s an unwritten rule that when a minister retires from the church, he needs to find another church to worship in because his presence in the congregation is a constant reminder of his past service and makes it harder for the new pastor to be successful.”

Experts agree a former president launching such a full-throated assault on his successor is unusual. It is also unusual for the current president to call the previous president “sick” and regularly accuse him of overseeing a massive illegal conspiracy, as Trump has done.

“It’s a hard job. He’s got plenty on his agenda. It’s difficult,” former President George W. Bush said in early 2009, explaining why he intended to abstain from criticizing Obama. “A former president doesn’t need to make it any harder.”

Despite that sentiment, Bush did publicly question Obama’s policies at times. In one June 2009 speech, he argued against Obama’s economic stimulus package and his efforts to close the Guantanamo Bay detention facility in Cuba, but he did not refer to Obama by name.

Historians often highlight the unexpected friendships and alliances forged by former presidents, but it is not unheard of for a sitting president to be overtly criticized by his predecessor. Herbert Hoover made his opposition to the policies of Franklin D. Roosevelt well-known, declaring in a 1936 speech that the New Deal was “the poisoning of Americanism” and calling for a “holy crusade” against it. He then actively supported Roosevelt’s opponent in the general election.

Harry Truman criticized Dwight Eisenhower sharply, and he delivered more than 20 campaign speeches on behalf of Democratic nominee Adlai Stevenson in 1956. He also campaigned “extensively” for Democrats in the 1958 midterms, according to the Library of Congress.

After Ronald Reagan accused Jimmy Carter of leaving the U.S. in “the worst economic mess since the days of Franklin Roosevelt” in 1982, Carter fired back, alleging at a Democratic National Committee fundraiser that Reagan was not yet ready to accept the responsibilities of the presidency.

Carter has become more vocal in recent years, slamming the war in Iraq regularly throughout George W. Bush’s presidency. In 2007, Carter declared Bush’s administration “the worst in history,” but he later said that comment was careless and not intended as a personal slight against Bush. Clinton and Obama have also drawn Carter’s ire at time.

If Obama is violating the unspoken rules of the post-presidency club over Trump, he is not alone. Clinton, who campaigned doggedly for his wife in 2016, has occasionally criticized Trump since the election as well. Carter has publicly called Trump “a disaster,” George W. Bush has made public statements widely perceived as anti-Trump, and George H.W. Bush was quoted as calling him a “blowhard” in a 2017 book.

As Obama hits the midterm campaign trail in a way none of his modern predecessors have, Timothy Blessing, a professor of history and political science at Alvernia University and co-author of “Greatness in the White House,” noted he is unique in one other significant way.

“If you go back to Teddy Roosevelt, he broke with the Republican Party and Republicans didn’t particularly want him to campaign for them,” he said. “Taft was a defeated president and persona non grata for Republicans. Wilson had a serious stroke. Harding was dead. Coolidge was very sick. Hoover was persona non grata because of the Depression. FDR was dead.”

Harry Truman left office with an approval rating in the 20s, Dwight Eisenhower had suffered heart attacks, John F. Kennedy was assassinated, Lyndon Johnson was sick, and Richard Nixon was “the ultimate persona non grata,” Blessing said. More recent presidents have also struggled with health problems or scandals tainting their image toward the end of their terms.

“It’s really hard to find presidents who are both popular and healthy after they leave the White House,” Blessing said.

After 20 months of President Trump discarding countless norms of presidential behavior, some shrugged off Obama’s divergence from expectations for post-presidential conduct.

“After Trump has stomped - proudly – on so many norms, traditions, and protocols, to accuse Obama of breaking some post-presidential decorum is just bizarre,” Ormsby said. “Of course, if he appears in tan suit, then folks are free to feel scandalized."

According to Sragow, too much is on the line in November for Democrats to withhold one of their most potent political weapons in deference to perceived traditions.

“Given how high the stakes are for the Democrats in this fall's congressional elections, the party leadership would be committing political malpractice if it didn't pull out all the stops,” he said.